In magnetic CRT deflection systems large energy values are involved in achieving the desired results. Unless a high-efficiency deflection system is employed, an inordinate amount of power must be expended in the deflection amplifier. It has been found that switching-type deflection generators produce the required deflection values while dissipating relatively little power. Accordingly such generators have come into wide use. However, while the switching circuits are highly useful they are very difficult to modulate so as to shape the deflection signals they produce. Since a switching-device is either off or fully on, any modulation of its output must be accomplished at high level, thus rendering sweep linearization and control very difficult.
The problems of sweep control are considerably aggravated in projection type CRT displays. In such devices a small CRT raster of great brightness is optically magnified and projected onto a viewing screen. Any magnification will clearly magnify any deflection errors. Therefore in projection type displays it is very important to be able to control the raster to produce an acceptable magnified picture.
Another problem in projection systems arises when the projector is not located perpendicular to the center of the viewing screen. Off-axis projection will produce a distorted raster shape and it is desirable to correct the deflection or raster scanning signals to offset such distortion.
Still another problem arises when a projection system involves more than one projector. If two or more rasters are to be projected, such as could be done in order to produce a color display, they will have to be converged so that all areas of each raster conform to the same areas of all of the other rasters. Clearly such convergence requires precision control of the shape of the scanning or deflection signals applied to the projectors.
The above-mentioned problems militate against high efficiency deflection systems and the typical approach is to use inefficient but controllable deflection amplifiers. This approach is wasteful of energy and the waste heat produced must be dissipated by some means. If the dissipation is not adequate, the amplifiers overheat and have short life. Also their excessive temperatures tend to degrade other nearby electronic components.